A wireless telecommunications system has been proposed in which a geographical area is divided into cells, each cell having one or more central terminals (CTs) for communicating over wireless links with a number of subscriber terminals (STs) in the cell. These wireless links are established over predetermined frequency channels, a frequency channel typically consisting of one frequency for uplink signals from a subscriber terminal to the central terminal, and another frequency for downlink signals from the central terminal to the subscriber terminal.
The system finds a wide variety of possible applications, for example in rural, remote, or sparsely populated areas where the cost of laying permanent wire or optical networks would be too expensive, in heavily built-up areas where conventional wired systems are at full capacity or the cost of laying such systems would involve too much interruption to the existing infrastructure or be too expensive, and so on.
In one embodiment, the central terminal may be connected to a telephone network and exists to relay messages from subscriber terminals in the cell controlled by the central terminal to the telephone network, and vice versa. By this approach, an item of telecommunications equipment connected to a subscriber terminal may make an outgoing call to the telephone network, and may receive incoming calls from the telephone network.
However, such a wireless telecommunications system is not restricted to use with telephone signals, but could instead handle any other appropriate type of telecommunications signal, such as video signals, or data signals such as those used for transmitting data over the Internet, and in order to support new technologies such as broadband and video-on-demand technologies.
FIG. 1 illustrates an example of a typical prior art configuration for a subscriber terminal for such a wireless telecommunications system. FIG. 1 includes a schematic representation of customer premises 22. A customer radio unit (CRU) 24 would typically be mounted on the customer's premises and may include a flat panel antenna or the like 23. The customer radio unit is mounted at a location on the customer's premises, or on a mast, etc, and in an orientation such that the flat panel antenna 23 within the customer radio unit 24 faces in the direction 26 of the central terminal for the service area in which the customer radio unit 24 is located.
The customer radio unit 24 is typically connected via a drop line 28 to a power supply unit (PSU) 30 within the customer's premises. The power supply unit 30 is connected to the local power supply for providing power to the customer radio unit 24 and to a network terminal unit (NTU) 32. The customer radio unit is also connected via the power supply unit 30 to the network terminal unit 32, which in turn is connected to telecommunications equipment in the customer's premises, for example to one or more telephones 34, facsimile machines 36 and computers 38. The telecommunications equipment is represented as being within a single customer's premises. However, this need not be the case, as the subscriber terminal 20 may support more than one line, so that a number of items of subscriber telecommunications equipment could be supported by a single subscriber terminal 20. The subscriber terminal 20 can also be arranged to support analog and digital communications, for example analog communications at 16, 32 or 64 Kbits/sec or digital communications in accordance with the ISDN BRA standard.
The CRU 24 typically includes all of the necessary processing circuitry to convert incoming wireless telecommunications signals into signals recognisable by the items of telecommunications equipment, and also to convert such signals from those items of telecommunications equipment into wireless telecommunications signals for transmission from the antenna 23.
A significant problem with this approach is that the CRU 24 is an expensive item of equipment to replace. Since this is generally located on the outside of the customer's premises, it is prone to theft. In addition, all of the components within the CRU 24 have to be able to withstand the exposure to varying climatic conditions that arise as a result of the CRU 24 being mounted externally. For example, the components must be able to withstand significant variations in temperature, and variations in humidity.
However, one reason why the CRU 24 has previously included all of the necessary processing circuitry to convert incoming wireless telecommunications signals into signals recognizable by the items of telecommunications equipment is that it reduces the technical complexity of the subscriber terminal to have all of the processing circuitry in one housing.
Further, problems with attenuation of the wireless signals transmitted between the central terminal and the subscriber terminal, and vice versa, have previously dictated that the processing circuits of the subscriber terminal should be located physically close to the antenna 23. To illustrate this, it will be appreciated that a signal transmitted from the central terminal at a predetermined power level will be attenuated as it is propagated to the antenna 23 of the subscriber terminal 20.
Once the signal has been received by the antenna 23, there will also be further attenuation within the subscriber terminal as the signal is passed from the antenna to the processing circuits within the subscriber terminal.
Clearly, the further away those processing circuits are from the antenna, then the greater the attenuation is likely to be. A signal strength threshold will be determined below which a signal cannot be processed by the processing circuits within the subscriber terminal 20. Hence, in order to improve the range of the wireless telecommunications system, it has been considered advisable to minimise the distance between the antenna 23 and the processing circuitry of the subscriber terminal provided to process that received signal.
The above requirements have led to the development of subscriber terminals such as those illustrated in FIG. 1, in which an expensive customer radio unit 24 engineered to withstand exposure to varying climatic conditions has been mounted on the exterior of a subscriber's premises.